In transformer design, the winding turn-ratio and the duty cycle of a switching power source determine the effective output power of a transformer. When the winding turn-ratio and the duty cycle are carefully designed, the conversion efficiency of the transformer can be optimized. A winding scheme of a conventional transformer usually includes an integer number of winding turns. Nevertheless, in some applications, the number of winding turns of a transformer must be non-integer for achieving an optimum efficiency design of switching modulation for a converter. The approach of a conventional transformer which has non-integer winding-turn usually leads to asymmetric distribution of the intensity of magnetic field and unbalanced magnetic flux, so that heat may result from an uneven magnetic field of a transformer core, and the overall conversion efficiency is degenerated.